Circle Execution
Circle is the current execution rail.
Paymod currently targets Circle's Developer-Controlled Wallets and USDC configured for the wallet. Circle executes and signs; Paymod's policy engine supplies the off-chain authorization in front of it - there is no on-chain contract of Paymod's own constraining the wallet the way an earlier Stellar-based design had.
Mainnet operation and Arc execution are not documented as live. The payment interface is chain-neutral, so a future Stellar, Solana or other rail does not change the Agent Wallet or policy model - only which infrastructure executes underneath it.